This invention relates in general to fluid control valves and in particular to an improved structure for a spring biased pilot pressure sub-assembly for such a fluid control valve.
In many hydraulic and pneumatic systems, control valves are provided for regulating the flow of fluid from a pressurized source to one or more controlled devices. Fluid control valves of this type generally include a case having a plurality of ports formed therein. A pressure port is provided which communicates with the pressurized source, while a tank port is provided which communicates with a fluid reservoir. One or more work ports are also provided which communicate with respective controlled devices. By selectively providing communication between the various ports, the operation of the controlled devices can be regulated in a desired manner.
For each of the work ports, a plunger valve assembly is typically provided within the case of the fluid control valve. Each of the plunger valve assemblies is operable to selectively provide communication between its associated work port and each of the pressure and tank ports. This is usually accomplished by means of an axially movable spool contained within the plunger valve assembly. The spool is movable upwardly and downwardly between opened and closed positions. In the opened position, the spool permits communication between the associated work port and the pressure port, thereby causing actuation of the controlled device. In the closed position, the spool provides communication between the associated work port and the tank port, thereby preventing actuation of the controlled device.
Axial movement of the spools is usually accomplished by means of a pivotable lever which is mounted on the upper end of the case. The lever is connected through respective linkages to each of the plunger valve assemblies. The lever is usually biased toward a center position. Pivoting movement of the lever in a first direction from the center position causes downwardly movement of one of the spools from the closed position to the opened position. Similarly, pivoting movement of the lever in a second direction from the center position causes downwardly movement of the other of the spools from the closed position to the opened position. The spools are usually biased upwardly by respective return springs toward the closed positions. These return springs typically react between spring seats formed on the case and portions of the associated linkages. As a result, an affirmative effort is required to pivot the lever from the center position so as to move the spools from their closed positions to their opened positions.
In fluid control valves of this type, it is often desirable to provide a mechanism whereby the lever can be pivoted within a limited range of movement from the center position without opening either of the plunger valve assemblies. This "dead band" range of lever pivoting movement is relatively small, plus or minus two degrees from the center position, for example. The purpose of the "dead band" range of movement is to prevent small movements of the lever from causing unintended movements of the spools and, therefore, operation of the controlled devices. Once the lever has been pivoted beyond the end of the "dead band" range, the spool is moved from the closed position to the opened position. When this occurs, there is a step increase in the magnitude of the fluid pressure supplied to the controlled device, from zero pressure to a predetermined initial step pressure. Further pivoting movement of lever causes a generally linear increase in the magnitude of the fluid pressure supplied to the controlled device from the initial step pressure to the maximum available system pressure.
To accomplish this "dead band" operation, it is known to provide a spring or similar resilient member in the linkage between the lever and each of the spools of the plunger valves. These springs (generally referred to as pilot springs) typically react between spring seats formed on the spools and portions of the associated linkages. Thus, when the lever is pivoted, the spool is not directly contacted so as to be moved downwardly to the opened position. Rather, the spool is biased by the pilot spring so as to be urged downwardly toward the opened position. The magnitude of the force exerted by the pilot spring determines the magnitude of the step increase in pressure discussed above. In other words, the magnitude of the initial step pressure is dependent upon the magnitude of the force exerted by the pilot spring. This spring biased structure for setting the initial step pressure is referred to as a pilot pressure sub-assembly for the fluid control valve.
The desired magnitude of the initial step pressure can vary from application to application for the fluid control valve. To accommodate this, means are usually provided in known pilot pressure sub-assemblies for adjusting the magnitude of the force exerted by the pilot spring. As mentioned above, the pilot springs typically react between spring seats formed on the spools and portions of the associated linkages. In the past, the adjustment of the force exerted by the pilot spring was accomplished by inserting and removing annular shims provided on the spring seats. By inserting and removing these shims, the distance separating the ends of the pilot spring (and, therefore, the spring force generated thereby) could be varied. While this method is effective, it has been found to be very time consuming. Also, it has been found to be difficult to accurately obtain a desired spring force. Accordingly, it would be desirable to provide an improved structure for a spring biased pilot pressure sub-assembly for a fluid control valve in which the force exerted by the pilot spring can be adjusted quickly and easily.